1. Field of the Invention
The present invention relates to a liquid crystal display device that achieves display by using an active-matrix liquid crystal panel.
2. Description of the Prior Art
Liquid crystal display devices (LCDs) find uses as displays in a variety of electronic devices and appliances for their compactness, low power consumption, and high display quality as compared with other types of display device such as cathode-ray tubes (CRTs) and plasma display panels (PDPs).
In a liquid crystal display device, when the driving of its liquid crystal panel is stopped, depending on the pattern that has been displayed thereon up to that moment, it sometimes takes an unduly long time for the electric charge accumulated in the liquid crystal panel to be discharged completely (hereinafter, this time will be referred to as the xe2x80x9caccumulated-charge discharge timexe2x80x9d). This not only causes an afterimage and thereby degrades display quality, but also, in some cases, leaves the electric charge accumulated in the liquid crystal panel and thereby leads to deterioration of the liquid crystal panel.
For this reason, various methods for shortening the accumulated-charge discharge time have been proposed to this date. Two of such methods will be described below. According to the first method, which applies to a transmissive liquid crystal panel, the backlight is kept lit for a predetermined time even after the driving of the liquid crystal panel has been stopped so that the accumulated-charge discharge time is shortened through the photoconductive effect of the switching devices that are connected between the liquid crystal layers of the liquid crystal panel and the signal lines.
According to the second method, as shown in FIG. 11, in the scanning electrode driving circuit 21 that drive the gates of the TFTs (thin-film transistors) connected to the liquid crystal layers of the liquid crystal panel 3, the line that supplies it with a voltage VEE for turning the TFTs off is connected through a resistor R to ground so that the accumulated-charge discharge time is shortened by the lower impedance with respect to ground.
Alternatively, as shown in FIG. 12, in the signal electrode driving circuit 22 that drives the signal lines of the liquid crystal panel 3, the lines that supply it with a plurality of voltages V1, V2, . . . , Vn are respectively connected through switches SW1, SW2, . . . , SWn and resistors R1, R2, . . . , Rn to ground, and these switches SW1, SW2, . . . , SWn are turned on when the supply of power is cut off so that the accumulated-charge discharge time is shortened by the lower impedance with respect to ground.
However, in a case where the driving of the liquid crystal panel needs to be stopped after the backlight is put out, or where the liquid crystal panel is of a reflective type and is not equipped with a backlight, the first method described above cannot be adopted but the second method described above is the only choice for the shortening of the accumulated-charge discharge time. Unfortunately, the second method cannot satisfactorily shorten the accumulated-charge discharge time, and thus cannot satisfactorily prevent the degradation of display quality and the deterioration of the liquid crystal panel.
Moreover, in the case of a liquid crystal panel provided with functions of both transmissive and reflective types (hereinafter, such a liquid crystal panel will be referred to as an xe2x80x9cadvanced liquid crystal panelxe2x80x9d), to adopt the first method, even when the liquid crystal panel is operating in the mode in which it is driven without lighting the backlight, the backlight needs to be lit temporarily every time the driving of the liquid crystal panel is stopped.
On the other hand, other methods for shorting the accumulated-charge discharge time and methods for preventing afterimages and disturbances in the displayed image are proposed in Japanese Patent Applications Laid-Open Nos. H10-222134, H11-212522, and H11-271707. However, according to these methods, when the driving of the liquid crystal panel is stopped, it is necessary to drive the scanning lines and signal lines of the liquid crystal panel, which requires complicated control.
An object of the present invention is to provide a liquid crystal display device that prevents degradation of display quality and deterioration of its liquid crystal panel through simple control even in a case where it is impossible to shorten the accumulated-charge discharge time by the use of a backlight.
To achieve the above object, according to the present invention, in a liquid crystal display device that achieves display by using an active-matrix liquid crystal panel, for a predetermined time after the driving of the liquid crystal panel is stopped, a voltage is kept applied only to the common electrode through which all liquid crystal layers constituting the liquid crystal panel are driven.
It has been confirmed that this configuration makes it possible to shorten the accumulated-charge discharge time to a degree comparable to a method using a backlight. It has also been confirmed that, in a case where the voltage that is kept applied to the common electrode of the liquid crystal panel for the predetermined time after the driving of the liquid crystal panel is stopped is a direct-current voltage, the greater the absolute value of this voltage, and the longer the time for which this voltage is kept applied to the common electrode after the driving of the liquid crystal panel is stopped, the more the accumulated-charge discharge time is shortened. Thus, according to the present invention, it is possible to prevent degradation of display quality and deterioration of the liquid crystal panel through simple control even in a case where it is impossible to shorten the accumulated-charge discharge time by the use of a backlight.
It has also been confirmed that it is possible, when the driving of the liquid crystal panel is stopped, to shorten the accumulated-charge discharge time by shifting at least one of the timing with which to stop the supply of the signal applied to the common electrode through which all liquid crystal layers constituting the liquid crystal panel are driven and the timing with which to stop the supply of the voltage that the circuit driving the liquid crystal panel requires to turn off the switching devices of the liquid crystal panel from the timing with which to stop the supply of the other signals and electric power related to the driving of the liquid crystal panel.